Since the development and use of prosthetic heart valves, numerous in vitro and in vivo techniques have been used to determine the performance of these valves. In vitro circulatory systems have been used to determine flow patterns, regurgitation, and pressure differences for a given valve. In the past years, Doppler echocardiography has become a useful, noninvasive, diagnostic method in the study of cardiac anatomy and physiology. An in vitro circulatory system capable of accepting heart valves in the aortic and mitral positions is utilized. This system is capable of reproducing physiologic flow rates and pressures. Doppler echocardiography will be used to determine the fluid velocity patterns of various prosthetic valves placed in the in vitro circulatory system. Pressure differences will be calculated from these velocities and compared to those pressures obtained from pressure transducers. Doppler color flow mapping data obtained from Doppler echocardiography will also be used to compare flow patterns obtained in vivo to those obtained for the same explanted valves in vitro.